In preparing an electrically conductive paste by a conventional technique, copper powder as an electrically conductive ingredient is dispersed along with a glass frit into an organic vehicle to prepare a paste. This electrically conductive paste is used to form electrically conductive circuit patterns. Such an electrically conductive circuit pattern is formed by a method in which the electrically conductive paste is first applied on a ceramic substrate by coating or printing, and the applied paste is dried and then baked in a non-oxidizing atmosphere such as nitrogen (N.sub.2) or in a weakly oxidizing atmosphere containing oxygen (O.sub.2) at a concentration of about 1,000 ppm or less.
It is important for electrically conductive pastes of this kind to inhibit the copper powder particles contained therein from undergoing oxidation. This is because if oxidation of the copper powder particles proceeds, the electrically conductive paste comes to have poor printing properties or the paste after baking has an increased specific resistance and impaired solderability. In order to avoid occurrence of such troubles, a technique of subjecting beforehand copper powder to an oxidation preventing treatment is employed. As one of such oxidation preventing treatment, a method has been employed, in which copper powder is immersed in a boric acid-containing solution, followed by mixing with stirring, and the copper powder particles are then filtered off and subjected to heating treatment at a temperature of from 50.degree. to 260.degree. C. to form a boric acid layer covering the surface of the copper powder particles. In preparing the boric acid-containing solution, an alcohol solvent, e.g., methanol, has been generally employed, because of its high ability of dissolving boric acid.
However, the conventional oxidation preventing method described above has a problem in that, in the case where a boric acid-containing solution used in the above method is prepared using an alcohol solvent in which boric acid has a high solubility, a volatile ester is formed between the alcohol solvent and boric acid, and the boric acid also evaporates during the mixing treatment with stirring and during the heating treatment. As a result, the oxidation prevention effect of boric acid becomes insufficient.